The invention relates to a d-c regulator circuit of a type related to the d-c regulator circuit shown in German Offenlegungsschrift No. 26 22 351.
With a conventional regulator circuit, the voltage of a given d-c voltage source can be reduced so that the mean value of the output voltage applied across a load can be controlled nearly down to zero. This mean value depends primarily on the duty cycle of the main valve, but due to the necessary reversal of the charge of the commutating capacitor, it is not possible for the duty cycle of the main valve to fall below a certain minimum value. During the quenching process, the voltage of the commutating capacitor is applied to the load in addition to the input voltage. As a result, minimum voltages no less than about 8 to 12% of the d-c input voltage can be obtained in the normal control range, in which the load voltage, i.e., the output voltage of the regulator, is controlled by the duty cycle of the main regulator valve.
In the previously-mentioned German Offenlegungsschrift No. 26 22 351 provision is made to extend the normal control to smaller output voltages. To do so, the main valve remains blocked and, during the reversal time, only a certain portion of the capacitor voltage is applied to the load through appropriate control of the commutating valve, the reversal valve, and the controlled swingback valve ("phase angle control"). As compared to conventional d-c regulator circuits, the circuit in the Offenlegungsschrift is distinguished by the polarity of the reversal valve and the swingback valve.
In the normal control range the circuit includes means so that, during the time between extinguishing and refiring the main valve, the commutating capacitor is held at the polarity it assumed due to the current commutated to the quenching branch during the quenching process. This state of the commutating capacitor is retained also after the main valve has been fired and the charge of the capacitor is reversed by firing the reversal valve only at a time, which is determined by the inductance of the reversing choke and the capacitance of the commutating capacitor, prior to the planned firing of the reversing valve, and the circuit is prepared for the quenching of the main valve. The quenching valve and the swingback valve are fired simultaneously. To reduce the output voltage, the time that the main valve conducts is reduced by shortening the time interval between the triggering of the main valve and the triggering of the reversing valve. This interval may be reduced more and more until both valves are triggered simultaneously.
To reduce the voltage still further, it is possible to use phase angle control, at which time the main valve no longer receives firing pulses. The output voltage is generated by the fact that the commutating valve carries current in the interval between the time the swingback valve is triggered and the time it ceases to conduct. As a result, a superposition of the voltage of the d-c voltage source and the voltage at the commutating capacitor is applied to the load. The lowering of the output voltage is thus accomplished by delaying the firing instant for the commutating valve, which must occur during the conduction period of the swingback valve, and shifting it toward the quenching instant of the swingback valve. As a result, the part of the capacitor voltage to be applied to the load is reduced.
However, it has been found, especially with operation in the phase angle control region, that the capacitor is overcharged considerably beyond the voltage required for quenching the main valve, or thyristor. Since the valves and chokes must be designed for the maximum reversal current and maximum inverse voltage, such overvoltages should be prevented. Therefore, Offenlegungsschrift provides a series circuit connected in parallel with the commutating valve and comprising a charging valve in series with an ohmic resistance, the charging valve being conductive to current of opposite polarity from the commutating valve so that the overcharge is partly returned to the d-c voltage source and is partly converted into heat. In order that, when the main valve is fired, the capacitor is kept from being partly discharged in the normal control range via the recharging branch and the capacitor charge is no longer sufficient for quenching the main valve, a modified pulse sequence is given in German Offenlegungsschrift No. 26 35 953 for the main control range, in which the reversing valve is always fired shortly before the main valve.
One disadvantage of such circuits is that, because of the recharging branch, much material and space are required and, because of the ohmic resistors, the circuits have lower efficiency.